Extract sc_delay_buffer

A video buffer had 2 responsibilities:
 - handle the frame delaying mechanism (queuing packets and pushing them
   after the expected delay);
 - keep only the most recent frame (using a sc_frame_buffer).

In order to reuse only the frame delaying mechanism, extract it to a
separate component, sc_delay_buffer.

app/meson.build

@@ -11,6 +11,7 @@ src = [
     'src/control_msg.c',
     'src/controller.c',
     'src/decoder.c',
+    'src/delay_buffer.c',
     'src/demuxer.c',
     'src/device_msg.c',
     'src/icon.c',